Chemical Engineering: An Introduction to Chemical Engineering Design

842 Pages·1983·17.791 MB·English

by  R.K. Sinnott

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C H E M I C AL ENGINEERING
VOLUME 6
An Introduction to Chemical Engineering Design
BY
R. K. SINNOTT
Department oJ Chemical Engineering, University College of Swansea
PERGAMON PRESS
Member of Maxwell Macmillan Pergamon Publishing Corporation
OXFORD NEW YORK BEIJING FRANKFURT
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Copyright © 1983 R. K. Sinnott
All Rights Reserved. No part of this publication may be reproduced,
stored in a retrieval system or transmitted in any form or by any
means: electronic, electrostatic, magnetic tape, mechanical, photo­
copying, recording or otherwise, without permission in writing from the
publishers.
First edition 1983
Reprinted with corrections 1985
Reprinted 1986, 1989
Reprinted with corrections 1991
Library of Congress Cataloging in Publication Data
Coulson, J. M. (John Metcalfe)
Chemical engineering.
Vol. published in Oxford, New York.
Vols. 1-2 lack series statement.
Includes bibliographical references and indexes.
Contents: v. 1. Fluid flow, heat transfer, and mass transfer—v. 2. Unit
operations v.6. Design by R. K. Sinnott.
I. Chemical engineering. I. Richardson, J. F. (John Francis)
II. Title. III. Series.
TP145.C78 660.2 54-14486
British Library Cataloguing in PubUcation Data
Coulson, J. M.
Chemical engineering.
Vol. 6: Design
1. Chemical engineering
I. Title II. Richardson, J. F.
III. Sinnott, R. K.
660.2 TP145.C78
ISBN 0-08-022969-7 (Hardcover)
ISBN 0-08-022970-0 (Flexicover)
Printed in Great Britain by BPCC Wheatons Ltd, Exeter
Author's Preface
THIS book has been written primarily for students on undergraduate courses in Chemical
Engineering and has particular relevance to their design projects. It should also be of
interest to graduates of other disciplines who are working in the chemical and process
industries. In writing it, I have drawn on my experience of teaching design at the
University College of Swansea, and on some years of experience in the process industries.
Books on design tend to fall into two categories. There are those written by academics,
that are largely philosophical discussions of the nature and methodology of the design
process, and which are usually of little practical use. And there are handbooks
(cookbooks) covering design methods, information, and data, which are often derided by
academics. As this book is intended to be used, the emphasis has been put on providing
useful design methods and techniques. Clearly, it is not possible to cover in detail in one
book the whole range of techniques and methods needed for the design of a chemical
manufacturing process. Nor could this be within the range of experience and expertise of
any one author. The approach taken has been to give sufficient detail for the preliminary
design of processes and equipment, and to back this up with references both to
authoritative texts and articles that cover the topics more thoroughly and to those that
give detailed design methods.
The explanations that are given of the fundamental principles underlying the design
methods are necessarily very brief. The scientific principles and unit operations of
Chemical Engineering are covered in Volumes 1, 2 and 3 of this work, and in other
textbooks cited in this volume, to which the reader is referred.
The chapters in this book can be grouped under three main topics. Chapters 1 to 9 and
14 cover process design, and include a brief explanation of the design method, including
considerations of safety, costing, and materials selection. Chapters 10, 11 and 12 cover
equipment selection and design. Chapter 13 covers the mechanical design of process
plant.
Chapters 1 to 12 can be used as a text for courses on process and equipment design,
omitting Chapter 2 and the first part of Chapter 3 for students who are familiar with
material and energy balance calculations. Chapter 13 will give Chemical Engineering
students some appreciation of the mechanical aspects of equipment design.
The art and practice of design cannot be learnt from books. The intuition and
judgement necessary to apply theory to practice will come only from practical experience.
I trust that this book will give its readers a modest start on that road.
In closing, I would like to express my appreciation to all those friends and colleagues
who have inñuenced my own development as a professional engineer, and so contributed
to this book.
R. K. S INNOTT
Preface
THE earlier volumes of this Series (Volumes 1, 2 and 3) dealt with the theoretical
background to chemical engineering processes and operations and with the functioning of
particular pieces of equipment. This volume completes the series and extends the
treatment of the subject, by showing how a complete process is designed and how it must
be fitted into the environment. It therefore includes material on flow-sheeting, piping,
mechanical construction, safety and costing. It relies heavily on the earlier volumes for a
discussion of the background theory, though in order to make the work complete in itself
it includes illustrations of equipment items which have already featured in the previous
works; furthermore, the treatment of distillation and heat exchanger design is expanded.
Whilst the book is directed primarily to undergraduate students of chemical engineering,
it should also be valuable to chemical engineers in industry (and particularly to those
studying for the Design Project) and to chemists and mechanical engineers who have to
tackle problems arising in the Process Industries.
The design engineer must use a wide range of information taken from a variety of
sources and must take into account many conflicting requirements—technical, economic
and environmental. Within the time span available to him, he must efl'ect a satisfactory
compromise and it should always be borne in mind that there is never a unique ''best"
solution to any design problem. Furthermore, what is a satisfactory design for one
location may be totally unsuitable elsewhere. Although it is impossible to convey a
complete philosophy through the medium of a single book, an attempt has been made to
make the reader aware of many of the diverse factors which must be incoφorated into any
one design. This volume provides only an introduction but gives an indication of sources
of more detailed information on individual branches of the subject.
J. F. RICHARDSON
J. M. COULSON
Acknowledgement
Material from British Standards is reproduced by permission of the British Standards
Institution, 2 Park Street, London WIA 2BS from whom complete copies of the
Standards can be obtained.
VI
Preface
THE earlier volumes of this Series (Volumes 1, 2 and 3) dealt with the theoretical
background to chemical engineering processes and operations and with the functioning of
particular pieces of equipment. This volume completes the series and extends the
treatment of the subject, by showing how a complete process is designed and how it must
be fitted into the environment. It therefore includes material on flow-sheeting, piping,
mechanical construction, safety and costing. It relies heavily on the earlier volumes for a
discussion of the background theory, though in order to make the work complete in itself
it includes illustrations of equipment items which have already featured in the previous
works; furthermore, the treatment of distillation and heat exchanger design is expanded.
Whilst the book is directed primarily to undergraduate students of chemical engineering,
it should also be valuable to chemical engineers in industry (and particularly to those
studying for the Design Project) and to chemists and mechanical engineers who have to
tackle problems arising in the Process Industries.
The design engineer must use a wide range of information taken from a variety of
sources and must take into account many conflicting requirements—technical, economic
and environmental. Within the time span available to him, he must efl'ect a satisfactory
compromise and it should always be borne in mind that there is never a unique ''best"
solution to any design problem. Furthermore, what is a satisfactory design for one
location may be totally unsuitable elsewhere. Although it is impossible to convey a
complete philosophy through the medium of a single book, an attempt has been made to
make the reader aware of many of the diverse factors which must be incoφorated into any
one design. This volume provides only an introduction but gives an indication of sources
of more detailed information on individual branches of the subject.
J. F. RICHARDSON
J. M. COULSON
Acknowledgement
Material from British Standards is reproduced by permission of the British Standards
Institution, 2 Park Street, London WIA 2BS from whom complete copies of the
Standards can be obtained.
VI
CHAPTER 1
Introduction to Design
1.1. Introduction
This chapter is an introduction to the nature and methodology of the design process,
and its appHcation to the design of chemical manufacturing processes.
1.2. Nature of design
This section is a general, somewhat philosophical, discussion of the design process; how
a designer works. The subject of this book is chemical engineering design, but the
methodology of design described in this section appHes equally to other branches of
engineering design.
Design is a creative activity, and as such can be one of the most rewarding and satisfying
activities undertaken by an engineer. It is the synthesis, the putting together, of ideas to
achieve a desired purpose. The design does not exist at the commencement of the project.
The designer starts with a specific objective in mind, a need, and by developing and
evaluating possible designs, arrives at what he considers the best way of achieving that
objective; be it a better chair, a new bridge, or for the chemical engineer, a new chemical
product or a stage in the design of a production process.
When considering possible ways of achieving the objective the designer will be
constrained by many factors, which will narrow down the number of possible designs;
but, there will rarely be just one possible solution to the problem, just one design. Several
alternative ways of meeting the objective will normally be possible, even several best
designs, depending on the nature of the constraints.
These constraints on the possible solutions to a problem in design arise in many ways.
Some constraints will be fixed, invariable, such as those that arise from physical laws,
government regulations, and standards. Others will be less rigid, and will be capable of
relaxation by the designer as part of his general strategy in seeking the best design. The
constraints that are outside the designer's influence can be termed the external
constraints. These set the outer boundary of possible designs; as shown in Fig. 1.1, Within
this boundary there will be a number of plausible designs bounded by the other
constraints, the internal constraints, over which the designer has some control; such as,
choice of process, choice of process conditions, materials, equipment.
Economic considerations are obviously a major constraint on any engineering design:
plants must make a profit.
Time will also be a constraint. The time available for completion of a design will usually
limit the number of alternative designs that can be considered.
The stages in the development of a design, from the initial identification of the objective
1
CHEMICAL ENGINEERING
Region of all designs
f
φ" \ ^
a
i
JSAOC)
'External' constraints
'Internal' constraints
FIG. 1.1. Design constraints
to the final design, are shown diagrammatically in Fig. 1.2. Each stage is discussed in the
following sections.
Figure 1.2 shows design as an iterative procedure; as the design develops the designer
will be aware of more possibilities and more constraints, and will be constantly seeking
new data and ideas, and evaluating possible design solutions.
Objective
(design
specification)
Collection
of data,
physical
properties
design
methods
Generation
of possible
desIign s
Selection
and
Evaluation
(optimisation )
Final
design
FIG. 1.2. The design process
INTRODUCTION TO DESIGN 3
1.2.1. The design objective {the need)
Chaddock (1975) defined design as, the conversion of an ill-defined requirement into a
satisfied customer.
The designer is creating a design for an article, or a manufacturing process, to fulfil a
particular need. In the design of a chemical process, the need is the public need for the
product, the commercial opportunity, as foreseen by the sales and marketing organis­
ation. Within this overall objective the designer will recognise sub-objectives; the
requirements of the various units that make up the overall process.
Before starting work the designer should obtain as complete, and as unambiguous, a
statement of the requirements as possible. If the requirement (need) arises from outside
the design group, from a client or from another department, then he will have to elucidate
the real requirements through discussion. It is important to distinguish between the real
needs and the wants. The wants are those parts of the initial specification that may be
thought desirable, but which can be relaxed if required as the design develops. For
example, a particular product specification may be considered desirable by the sales
department, but may be difficult and costly to obtain, and some relaxation of the
specification may be possible, producing a saleable but cheaper product. Whenever he is
in a position to do so, the designer should always question the design requirements (the
project and equipment specifications) and keep them under review as the design
progresses.
Where he writes specifications for others, such as for the mechanical design or purchase
of a piece of equipment, he should be aware of the restrictions (constraints) he is placing
on other designers. A tight, well-thought-out, comprehensive, specification of the
requirements defines the external constraints within which the other designers must work.
1.2.2. Data collection
To proceed with a design, the designer must first assemble all the relevant facts and data
required. For process design this will include information on possible processes,
equipment performance, and physical property data. This stage can be one of the most
time consuming, and frustrating, aspects of design. Sources of process information and
physical properties are reviewed in Chapter 8.
Many design organisations will prepare a basic data manual, containing all the process
"know-how" on which the design is to be based. Most organisations will have design
manuals covering preferred methods and data for the more frequently used, routine,
design procedures.
The national standards are also sources of design methods and data; they are also
design constraints.
The constraints, particularly the external constraints, should be identified early in the
design process.
1.2.3. Generation of possible design solutions
The creative part of the design process is the generation of possible solutions to the
problem (ways of meeting the objective) for analysis, evaluation and selection. In this
activity the designer will largely rely on previous experience, his own and that of others. It
4 CHEMICAL ENGINEERING
is doubtful if any design is entirely novel. The antecedence of most designs can usually be
easily traced. The first motor cars were clearly horse-drawn carriages without the horse;
and the development of the design of the modern car can be traced step by step from these
early prototypes. In the chemical industry, modern distillation processes have developed
from the ancient stills used for rectification of spirits; and the packed columns used for gas
absorption have developed from primative, brushwood-packed towers. So, it is not often
that a process designer is faced with the task of producing a design for a completely novel
process or piece of equipment.
The experienced engineer will wisely prefer the tried and tested methods, rather than
possibly more exciting but untried novel designs. The work required to develop new
processes, and the cost, is usually underestimated. Progress is made more surely in small
steps. However, whenever innovation is wanted, previous experience, through prejudice,
can inhibit the generation and acceptance of new ideas; the ''not invented here"
syndrome.
The amount of work, and the way it is tackled, will depend on the degree of novelty in a
design project.
Chemical engineering projects can be divided into three types, depending on the novelty
involved:
1. Modifications, and additions, to existing plant; usually carried out by the plant
design group.
2. New production capacity to meet growing sales demand, and the sale of estabhshed
processes by contractors. Repetition of existing designs, with only minor design
changes.
3. New processes, developed from laboratory research, through pilot plant, to a
commercial process. Even here, most of the unit operations and process equipment
will use established designs.
The first step in devising a new process design will be to sketch out a rough block
diagram showing the main stages in the process; and to list the primary function
(objective) and the major constraints for each stage. Experience should then indicate what
types of unit operations and equipment should be considered. Jones (1970) discusses the
methodology of design, and reviews some of the special techniques, such as brainstorming
sessions and synectics, that have been developed to help generate ideas for solving
intractable problems.
A good general reference on the art of problem solving is the classical work by Polya
(1957).
The generation of ideas for possible solutions to a design problem cannot be separated
from the selection stage of the design process; some ideas will be rejected as impractical as
soon as they are conceived.
L2,4. Selection
The designer starts with the set of all possible solutions bounded by the external
constraints, and by a process of progressive evaluation and selection, narrows down the
range of candidates to find the ''best" design for the purpose.
The selection process can be considered to go through the following stages: